1. Field of the Invention
The present disclosure relates to the novel preparation of polymer monoliths for solvent exchange in a multistep reaction, for example, a method for the exchange and activation of fluoride ions on a flow through microfluidic chip for subsequent chemical synthesis.
2. Description of the Related Art
Methods in the current state of the art relating to the exchange and activation of fluoride ions typically utilize micrometer sized beads or resins (50-100 um) as a solid phase support with different functionalities some methods result in the release of a [18F]fluoride ion in as little as 1% water in acetonitrile and further demonstrate radiofluorination of common substrates with moderate fluorination efficiency without performing additional azeotropic distillation. Other organizations have reported functionalizing the conventional beads with a long alkyl chain of quaternary ammonium which enables efficient recovery of the [18F]fluoride ion in low water content organic solution. This is set forth in J. Aerts et al., Fast Production of Highly Concentrated Reactive [18F] Fluoride for Aliphatic and Aromatic Nucleophilic Radiolabeling, Tetrahedron Letters 51, 64 (2010), which is hereby incorporated herein in its entirety by reference.
The above described methods demonstrated successful radiofluorination of several substrates as a model system with moderate yield without performing additional drying steps. However, both methods utilized micron-sized beads with large bed volume packed in a cartridge format, which are conventionally used in macroscale synthesis. Such an approach can not be efficiently downscaled to a microfluidic platform due to the limited numbers of beads that can be placed in microchannels and thus a limited availability of surface active sites for ion exchange. For example, one of the major limitations of fluoride ion concentration utilizing a microfluidic chip using conventional resins is the inability to trap a sufficient number of radioactive fluoride ions within the micron-sized channels. Such a convention method is set forth in C. C. Lee et. al., Multistep Synthesis of a Radiolabled Imaging Probe Using integrated Microfluidics, Science 310, 179.3 (2005), which is hereby incorporated herein in its entirety by reference. The procedure and techniques using microchips as reported here can overcome such challenges.
In the area of microfluidic devices for chemical synthesis, the state of the art had previously involved solid supports utilizing the immobilization of micrometer sized resin within microfluidic channels for various applications such as biochemical transformation, catalytic reactions and solid phase extraction. More recently, the development of polymeric monolith materials have demonstrated enhanced performance over their resin counterparts because of the polymeric monolith's unique characteristics, such as ease of preparation, versatile chemistries and functionalities, controlled surface area, large through pores and high hydrodynamic flow. A polymer monolith as used herein includes a continuous polymer bed, macroporous polymer membranes, continuous polymer rods, porous silica rods, a polymer continuous column support, the specifically defined materials and monolith embodiments described herein and or any polymeric structure constituting or acting as a single, often rigid, uniform whole.
Specifically for microfluidic applications, the number of loading sites or total active surface area is extremely critical in determining the yield and efficiencies. For example, scientists at the University of Hull packed 60-100 μm sized polystyrene beads functionalized with bicarbonate anions within a microfluidic chip with dimensions of 30,000 μm×5,000 μm×250 μm. This is set forth in F. De Leonardis et al., On-Chip Pre-Concentration and Complexation of [18F] Fluoride Ions via Regenerable Anion Exchange Particles for Radiochemical Synthesis of Positron Emission Tomography Tracers, Journal of Chromatography A, 1218, 4714 (2011), which is hereby incorporated herein in its entirety by reference. Large dimension microfluidic channels are needed to facilitate packing sufficient numbers of beads (with sufficient surface area) for fluoride ion trapping. Due to the large bead volume, an average of 500 μL of phase transfer catalyst solution is needed to efficiently elute the totality of fluoride ions from the polymer support. This methodology yielded only a 2-fold volume concentration (1000 μL from the cyclotron water to 500 μL of the pre-concentrated fluoride ion). While such volume is sufficient for most macroscale synthesis, the 500 μL of radioactivity cannot be efficiently loaded to a batch microfluidic device for radiosynthesis (average reaction volume: 5-20 μL).
Another flow through microfluidic method for concentrating and drying of [18F] ions is via an electrochemical method, which relies on electrical potential to trap the negatively charged fluoride ion onto one of the electrodes. The [18F]fluoride is then released in an aprotic solvent by reversing the applied potential for subsequent radiofluorination reaction. Such methods are further set forth and described in detail in the following: 1) H. Saiki et al, Electrochemical Concentration of No-Carrier-Added [18F]Fluoride from [18O]Water in a Disposable Microfluidic Cell for Radiosynthesis of 18F-Labeled Radiopharmaceuticals, Applied Radiation and Isotopes, 68, 1703 (2010); 2) R. Wong et al., Reactivity of Electrochemically Concentrated Anhydrous [18F]Fluoride for Microfluidic Radiosynthesis of 18F-Labeled Compounds, Applied Radiation and Isotopes 70,193 (2012); 3) J. I. Morelle, et al. ED. (2008), vol. EP20060447128; all three of said documents hereby incorporated herein in their entirety by reference. Another example of such methods is the demonstrated successful radiofluorination of common positron emission tomography (PET) probes by after electrochemical trapping of the fluoride ion. Said system relies on the ability to prepare an anhydrous phase transfer catalyst, such as a cryptand, for example, Kryptofix® and potassium bicarbonate (K2.2.2/KHCO3) in acetonitrile to release the trapped fluoride from the electrochemical cell. Such an electrochemical platform requires high voltage and other electrical auxiliaries for operation, and thus may not be easily integrated to existing flow through chemistry.
The unstable [18F]fluorine isotope plays an important role in radiopharmaceuticals as an ideal probe for positron emission tomography (PET) imaging. PET imaging is an in-vivo, non-invasive imaging technique that uses radiolabelled compounds (in tracer quantities) to measure biochemical, biological, and pharmaceutical processes with high sensitivity. Examples of PET imaging processes are set forth in M. E. Phelps, PET: Molecular Imaging and Its Biological Applications (Springer, 2004), which is hereby incorporated herein in its entirety by reference.
This powerful imaging technique has enabled biologists and physicians to diagnose emergence of cancer or other diseases at an early stage by identifying biological changes and to differentiate between benign and malignant lesions to improve prognosis and lower the cost of therapy, personalizing therapies and stage the effectiveness of therapeutic methods, thus increasing the rate of survival, and developing effective drugs and therapeutic methods to prevent and cure cancer.
Fluorine-18 possesses many desirable properties such as the strong and stable C—F bond and the relatively low energy (max: 0.645 MeV) resulting in a short linear range and thus providing high image resolution. These and other properties of Fluorine-18 are set forth in M. C. Lasne et al., Contrasting Agents II, W. Krause ED. (Springer Berlin Heidelberg, Berlin, Heidelberg, 2002), vol. 222, pp. 201-258, which is hereby incorporated herein in its entirety by reference.
Furthermore, the 109 minute half-life of Fluorine-18 provides sufficient time for the radiolabelled probe generated in one facility to be shipped to other nearby imaging centers and permits longer imaging protocols to investigate processes of slower kinetics.
Due to the efficacy of PET imaging, there has been a drastic increase in the growth of PET imaging in preclinical and clinical research, pharmaceuticals and medical communities. An example of such research is set forth in P. Y. Keng, et al, Positron Emission Tomography-Current Clinical and Research Aspects C. H. Hsieh, Ed. (InTech, 2012), which is hereby incorporated herein in its entirety by reference. However, the current bottle-neck in advancing PET imaging is in increasing efficiency and diversity of radiolabeling small molecules and biological compounds.
To date, only a few selected PET probes (i.e.: [18F]FDG and [18]NaF) are available at centralized radiopharmacies, and the availability of other radiolabeled compounds is severely limited by the cost, speed and efficiency of radiosynthetic method. Recently, microfluidic radiosynthesizers have emerged as a potential microscale radiosynthesizer for on-demand production of individual doses of PET probes as a technological tool in enabling researchers to synthesize both known and new PET probes of interest on demand. An example of this is demonstrated in A. M. Elizarov, Microreactors for Radiopharmaceutical Synthesis, Lab on a Chip, 9, 1326 (2009), which is hereby incorporated herein in its entirety by reference
In particular, continuous flow microfluidic and capillary microreactor radiosynthesizers have demonstrated remarkable reaction kinetics down to nine seconds to achieve moderate fluorination efficiency of mannose triflate. Examples of such results are set forth in J. M. Gillies et al., Microfluidic Reactor for the Radiosynthesis of PET Radiotracers, Applied Radiation and Isotropes 64, 325 (2006), which is hereby incorporated herein in its entirety by reference. Continuous-flow synthesizer platforms have several other advantages, such as the ability to conduct reactions at high temperature and/or high pressure, and to perform multiple optimization reactions in a short time. Examples of these and other advantages are set forth in H. J. Wester, et al, Fast and Repetitive In-Capillary Production of [18F]FDG, European Journal of Nuclear Medicine and Molecular Imaging 36, 653 (2008), which is hereby incorporated herein in its entirety by reference.
However, one of the major drawbacks of these previous continuous flow systems is their inability to perform solvent exchange and evaporation, which is one of the most critical processes of nucleophilic fluorination reactions. In these reactions, the radioactive F18-fluoride ion is generated in a cyclotron and then solvated by [18O]H2O. In water, the fluoride ion is inactive in relation to any chemical reaction. Therefore, the first step in a typical F18 radiosynthesis involves the concentration of the F18-fluoride ion, followed by several cycles of azeotropic distillation in the presence of suitable phase transfer catalyst to remove all the water, this process is commonly known as solvent exchange. To date, flow through synthesizer systems rely on other means of evaporation to remove the water from the [18]F-/[18O]H2O source, such as integrating to a macroscale synthesizer with an evaporation vessel. The dried and activated [18F]F-complex is then transferred to the microfluidic or capillary microreactor for subsequent fluorination steps. Such a flow-through synthesis system is set forth in G. Pascali, et al., Microfluidic Approach for Fast Labeling Optimization and Dose-On-Demand Implementation, Nuc. Med. Biol. 37, 547 (2010), which is hereby incorporated herein in its entirety by reference.
The integration of the milliliter flask reactor to microvolume procedures often resulted in the need to use a large volume of solvent to transfer the radioactive source. The low mass of [18F]fluoride often leads to considerable radioactivity losses due to surface adsorption on the walls of the typical Pyrex® vials used in most laboratories. Secondly, the need for an additional macroscale “drying” platform significantly increased the overall size of the radiosynthesizer platform, which defeats the inherent advantages of a compact-sized microreactor. Accordingly there is a need for practical and efficient interfaces for concentrating, drying and activating a non-carrier added (n.c.a.) [18F]fluoride solution generated in a cyclotron to provide of microliter volumes for F18-labeled radiopharmaceutical production on microfluidic platforms.